The Effect of Heat Treatment after Hydrothermal Reaction on the Lithium Storage Performance of a MoS2/Carbon Cloth Composite

In this study, 1T phase MoS2 nanosheets were synthesized on the surface of a carbon cloth via a hydrothermal reaction. After heat treatment, the 1T phase MoS2 was transformed into the 2H phase with a better capacity retention performance. As an anode material for lithium-ion batteries, 2H phase MoS2 on the carbon cloth surface delivers a capacity of 1075 mAh g−1 at a current density of 0.1 A g−1 after 50 cycles; while the capacity of the 1T phase MoS2 on the surface of the carbon cloth without heat treatment fades to 528 mAh g−1. The good conductivity of a carbon cloth substrate and the separated MoS2 nanosheets help to increase the capacity of MoS2 and decrease its charge transfer resistance and promote the diffusion of lithium ions in the electrode.


Introduction
Since the invention of lithium-ion batteries (LIBs) in 1991, they have become the primary power source for portable electronic devices due to their high-energy density, good cycling stability, and long service life [1,2].In recent years, due to the development of electric vehicles, material scientists have been required to develop LIBs with higher energy density [3][4][5].However, the currently commercialized graphite anode material cannot meet the increasing energy density requirements of LIBs because of its low theoretical capacity (372 mAh g −1 ) [6].Transition metal sulfides with a higher theoretical capacity than the graphite anode material have attracted considerable attention [7][8][9].As one of transition metal sulfides, MoS 2 has a theoretical specific capacity (670 mAh g −1 ), which is 1.8 times the theoretical capacity of graphite [10][11][12].MoS 2 has a layered structure.Single-layer MoS 2 consists of two layers of sulfur atoms sandwiched by a layer of molybdenum atoms [13].Multilayer MoS 2 is composed of several single-layer MoS 2 connections, with a spacing of ~0.65 nm between layers.The layered structure of MoS 2 facilitates the diffusion of lithium ions in the electrode and buffers the volume change during charge-discharge, which leads to its good cyclic reversibility [14,15].However, as an inorganic material, MoS 2 has poor conductivity.Ammonium molybdate, which is commonly used as a raw material for the hydrothermal synthesis of MoS 2 , is readily soluble in water but not in organic solvents.Due to the strong polarity of water, MoS 2 synthesized with the hydrothermal method is prone to agglomerate.Large-sized aggregates will decrease the sites of electrochemical reactions and increase the diffusion distance of lithium ions in the electrode, resulting in a reduction in their electrochemical performance.Therefore, determining how to improve the conductivity of MoS 2 and how to synthesize its nanostructures is an important strategy to enhance its electrochemical performance.
Carbonaceous materials have good conductivity, and their volume effect during charge-discharge is small [16,17].Thus, as substrates, carbonaceous materials can improve the conductivity of transition metal sulfides and buffer their volume changes.A carbon cloth is a flexible carbonaceous material woven with carbon fibers [18].Its good conductivity and mechanical flexibility make it an appropriate collector for flexible LIB electrodes.
Recently, there are some studies on the synthesis and lithium storage performance of MoS 2 /carbon cloth composites.Some effective strategies, such as the preparation of ultra-thin MoS 2 nanosheets, the fabrication of 3D graphene/MoS 2 spherical heterostructure [19], the N doping of MoS 2 [20], the preparation of CdS@MoS 2 core-shell structured nanospheres [21], and the P doping of MoS 2 [22] have been conducted to improve the lithium storage performance of MoS 2 /carbon cloth composites.However, the effect of heat treatment after the hydrothermal reaction on their lithium storage performance has not been the subject of investigation in this study.Heat treatment will transform 1T phase MoS 2 with a larger volume effect into 2H phase MoS 2 with a smaller volume effect, which is beneficial for improving its capacity retention performance.Herein, MoS 2 nanosheets were grown on the surface of the carbon cloth using a hydrothermal reaction and subsequent heat treatment was conducted to increase their electrochemical performance.

Material Synthesis
Carbon cloth (WOS1011, CeTech Co., Ltd., Taiwan, China) was pretreated with 1M nitric acid (Shanghai Titan Scientific Co., Ltd., Shanghai, China) aqueous solution for 3 h, washed several times with deionized water, and then dried at 60 • C for 12 h.Ammonium molybdate tetrahydrate (0.3 g) (Shanghai Titan Scientific Co., Ltd., Shanghai, China) and thiourea (Shanghai Titan Scientific Co., Ltd., Shanghai, China) with the S/Mo molar ratios of 4, 5, and 6 were weighed and then dissolved in 35 mL deionized water under magnetic stirring for 30 min to prepare the solution.The obtained solution and 4 pieces of pretreated carbon cloth (10 mm × 10 mm) were transferred to a Teflon-lined, stainless-steel autoclave (50 mL) and kept at 180 • C for 6 h.After being cooled to room temperature, the carbon cloth with MoS 2 nanosheets was washed several times with deionized water.Subsequently, the washed carbon cloth was dried at 60 • C for 12 h.The MoS 2 /carbon cloth composite synthesized with hydrothermal reaction was denoted as MoS 2 /CC-No HT.The MoS 2 /CC-No HT was placed in the tube furnace under Ar atmosphere, then sintered at 500 • C and maintained for 2 h with a heating rate of 3 • C min −1 .The obtained MoS 2 /carbon cloth composite was denoted as MoS 2 /CC-HT.For comparison, MoS 2 powder was synthesized without the addition of carbon cloth.The MoS 2 powder synthesized with hydrothermal reaction was denoted as MoS 2 -No HT.The MoS 2 powder synthesized with hydrothermal reaction and subsequent heat treatment was denoted as MoS 2 -HT.

Material Characterizations
X-ray diffractometer (XRD, Panalytical X'Pert3 Powder, PANalytical B.V., Almelo, The Netherlands) was utilized to measure crystal structure.X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze the chemical state and elemental valence of the samples.Scanning electron microscope (SEM, Hitachi S4800, Hitachi, Tokyo, Japan) and transmission electron microscope (TEM, JEOL JEM-F200, JEOL, Tokyo, Japan) were investigated and used to conduct the morphological observations and microstructural analysis of the samples.An ASAP2460 (Micromeritics, Atlanta, GA, USA) instrument was used to test the specific surface area and pore structure.

Electrochemical Measurements
The electrochemical performance of the composites was assessed utilizing CR2032type half coin cells.The coin cells were assembled in a glove box with both H 2 O and O 2 contents below 0.1 ppm.MoS 2 /CC-HT and MoS 2 /CC-No HT were utilized as working electrodes without any intervening steps.The weight of the active substances loaded on the carbon cloth was calculated to be approximately 2.3 ± 0.2 mg cm −2 .For MoS 2 -HT and MoS 2 -No HT powders, it is necessary to fabricate working electrodes before the assembly of coin cells.The powder samples were prepared as a uniform and stable slurry by mixing the active materials, carbon black, and polyvinylidene fluoride (PVDF) in N-methyl-2pyrrolidone (NMP) solvent with a weight ratio of 8:1:1.The slurry was applied to the surface of copper foil and dried at 60 • C for 12 h in a vacuum oven.Lithium foil was used as the counter electrode, while the Celgard 2400 membrane functioned as the separator.The electrolyte was 1.0 M LiPF 6 in a mixed solution that blended ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) at a volume ratio of 1:1:1.The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on the CHI660E (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) electrochemical workstation.The galvanostatic charge/discharge test was investigated with a Neware-CT3008 instrument (NEWARE Technology Limited, Shenzhen, China).

Results and Discussion
Figure 1 shows the SEM images of MoS 2 /CC-HT synthesized under different S/Mo molar ratios.The S/Mo molar ratio has a significant effect on the morphology of MoS 2 /CC-HT.As exhibited in Figure 1a,b, it can be seen that the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 5 is ultra-thin nanosheets.In Figure 1c, the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 6 shows a similar morphology to the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 5. From Figure 1d, it can be observed that the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 4 is composed of particles with the size of 50-200 nm and flower-like nanostructure assembled by nanosheets.Ultra-thin nanosheets will provide many electrochemical reaction sites, decrease the diffusion distance of lithium ions, and reduce the absolute volume change caused by lithiation-delithiation due to their large specific surface area and ultra-thin thickness.Therefore, in the following results and discussion, MoS    , respectively (Supplementary Materials). Figure 3b shows the XPS high-resolution spectrum of Mo 3d.The peaks at 232.76 and 229.61 eV are related to Mo 3d3/2 and Mo 3d5/2 of Mo 4+ in MoS2, respectively [28,29]; the peaks at 234.03 and 230.98 eV assigned to Mo 3d3/2 and Mo 3d5/2 of Mo 6+ are related to the Mo-O-C bond between carbon fibers and MoS2 nanosheets [23,30]; the peak at 226.78 eV corresponds to S 2s in MoS2. Figure 3c shows the high-resolution XPS spectrum of S 2p; the separated peaks at 163.59 and 162.40 eV correspond to S 2p1/2 and S 2p3/2 of S 2-, respectively [31].In Figure 3d, the XPS high-resolution spectrum of C 1s exhibited two separated peaks at 284.80 and 285.77eV, which are assigned to C-C and C-O bonds, respectively [28,32].002) and (100) crystal planes of graphitic carbon, respectively.These characteristic XRD peaks are from the carbon cloth [23][24][25].In the XRD pattern of MoS 2 /CC-No HT, the peak detected at 9.4 • is assigned to the (002) crystal plane of 1T phase MoS 2 [26,27].The peak intensity is weak, indicating the low crystallinity of the MoS 2 .In the XRD pattern of MoS 2 /CC-HT, the peaks observed at 13.8, 33.6, and 59.5 • are attributed to the (002), (100), and (110) crystal planes, respectively, of 2H phase MoS 2 (JCPDS No. 37-1492).The increase in peak intensity indicates an increase in the crystallinity of the 2H phase MoS 2 .According to the Scherrer equation, the average grain size of the 2H phase MoS 2 is 9.9 nm.The grain boundaries are rich in defects which provide channels for the diffusion of lithium ions.Thus, the ultrafine average grain size is conducive to the diffusion of lithium ions in the electrode.
X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical states and surface elemental composition of MoS 2 /CC-HT.As shown in Figure 3a, the XPS survey spectrum of MoS 2 /CC-HT, the spectrum exhibits characteristic peaks of Mo 3d, S 2p, C 1s, and O 1s, indicating the existence of MoS 2 and carbon in MoS 2 /CC-HT.The characteristic peaks of O 1s are attributed to the water absorbed on the surface of MoS 2 /CC-HT.The center, FWHM, and area percentage for the deconvoluted peaks of Mo 3d, S 2p, and C 1s are presented in Tabs.S1 to S3, respectively (Supplementary Materials). Figure 3b shows the XPS high-resolution spectrum of Mo 3d.The peaks at 232.76 and 229.61 eV are related to Mo 3d 3/2 and Mo 3d 5/2 of Mo 4+ in MoS 2 , respectively [28,29]; the peaks at 234.03 and 230.98 eV assigned to Mo 3d 3/2 and Mo 3d 5/2 of Mo 6+ are related to the Mo-O-C bond between carbon fibers and MoS 2 nanosheets [23,30]; the peak at 226.78 eV corresponds to S 2s in MoS 2 .Figure 3c shows the high-resolution XPS spectrum of S 2p; the separated peaks at 163.59 and 162.40 eV correspond to S 2p 1/2 and S 2p 3/2 of S 2− , respectively [31].In Figure 3d, the XPS high-resolution spectrum of C 1s exhibited two separated peaks at 284.80 and 285.77eV, which are assigned to C-C and C-O bonds, respectively [28,32].The TEM, HRTEM, and SAED analyses of MoS2 in MoS2/CC-HT are shown in Figure 4.The MoS2 in MoS2/CC-HT exhibits a sheet-like structure (Figure 4a), and the lattice fringes of MoS2 can be observed at its edges (Figure 4b). Figure 4c   The TEM, HRTEM, and SAED analyses of MoS 2 in MoS 2 /CC-HT are shown in Figure 4.The MoS 2 in MoS 2 /CC-HT exhibits a sheet-like structure (Figure 4a), and the lattice fringes of MoS 2 can be observed at its edges (Figure 4b). Figure 4c  The specific surface area and pore structure analysis of MoS2/CC-HT and the carbon cloth are demonstrated in Figure 5. From Figure 5a, it can be seen that the N2 adsorptiondesorption isotherms of MoS2/CC-HT and the carbon cloth belong to Type IV [33].Calculated with the BET method, the specific surface areas of MoS2/CC-HT and the carbon cloth are 2.41 and 0.86 m 2 g −1 , respectively [33].By removing the influence of the carbon cloth, it The specific surface area and pore structure analysis of MoS 2 /CC-HT and the carbon cloth are demonstrated in Figure 5. From Figure 5a, it can be seen that the N 2 adsorptiondesorption isotherms of MoS 2 /CC-HT and the carbon cloth belong to Type IV [33].Calculated with the BET method, the specific surface areas of MoS 2 /CC-HT and the carbon cloth are 2.41 and 0.86 m 2 g −1 , respectively [33].By removing the influence of the carbon cloth, it can be estimated that the specific surface area of MoS 2 in MoS 2 /CC-HT is ~10.16 m 2 g −1 .
From Figure 5b, it can be seen that MoS 2 /CC-HT has a rich micro-porous structure.The large specific surface area and rich micro-porous structure will increase electrochemical reaction sites and promote the lithium-ion diffusion in the electrode.The specific surface area and pore structure analysis of MoS2/CC-HT and the carbon cloth are demonstrated in Figure 5. From Figure 5a, it can be seen that the N2 adsorptiondesorption isotherms of MoS2/CC-HT and the carbon cloth belong to Type IV [33].Calculated with the BET method, the specific surface areas of MoS2/CC-HT and the carbon cloth are 2.41 and 0.86 m 2 g −1 , respectively [33].By removing the influence of the carbon cloth, it can be estimated that the specific surface area of MoS2 in MoS2/CC-HT is ~10.16 m 2 g −1 .From Figure 5b, it can be seen that MoS2/CC-HT has a rich micro-porous structure.The large specific surface area and rich micro-porous structure will increase electrochemical reaction sites and promote the lithium-ion diffusion in the electrode.MoS 2 /CC-HT is composed of MoS 2 and a carbon cloth.Thus, the electrochemical reactions related to its charge-discharge can be described by the following equations [23,28,32]: (1) Li Figure 6a demonstrates the CV curves of the carbon cloth.In the first scanning, the cathode peak around 0.01 V corresponds to Li + insertion into the carbon cloth and the formation of solid electrolyte interface (SEI) film, and the anode peak at 0.48 V corresponds to Li + de-insertion in the carbon cloth [34].In the second scanning, the cathode peaks at 0.01, 0.27, and 0.69 V are related to Li + insertion of the carbon cloth, and the anode peak at 0.38 V is related to Li + de-insertion of the carbon cloth.In the third scanning, the cathode peaks at 0.01, 0.19 and 0.81 V can be assigned to Li + insertion of the carbon cloth, and the anode peak at 0.38 V can be assigned to Li + de-insertion of the carbon cloth.Figure 6b demonstrates the CV curves of MoS 2 /CC-HT.In the first cathode scanning, the two cathode peaks appear at 1.52 and 1.02 V, which are attributed to the insertion of Li + into the MoS 2 layer to form Li x MoS 2 [35]; the cathode peak at 0.53 V is attributed to the formation of an SEI film, accompanied by the decomposition of Li x MoS 2 to form metal Mo and Li 2 S [36], whereas the cathode peak at 0.01 V is attributed to the intercalation of Li + in the carbon cloth.In the first anode scanning, the anode peak at 0.26 V is related to the de-intercalation Li + in the carbon cloth, while the anode peak at 2.19 V is related to the oxidation process of LiS 2 to S and Mo to MoS 2 [37].The CV curves of the second and third cycles almost coincide with each other, indicating that MoS 2 /CC-HT has good cyclic repeatability.In the second and third scanning, the cathode peak related to the formation of Li x MoS 2 by the insertion of Li + in MoS 2 layers shifts to 1.93 V; the cathode peak related to the decomposition of Li x MoS 2 to metallic Mo and Li 2 S shifts to 1.12 V [38]; the anode peaks at 0.01, 0.21, and 0.82 V can be assigned to the Li + insertion of the carbon cloth.
cathode peaks appear at 1.52 and 1.02 V, which are attributed to the insertion of Li + into the MoS2 layer to form LixMoS2 [35]; the cathode peak at 0.53 V is attributed to the formation of an SEI film, accompanied by the decomposition of LixMoS2 to form metal Mo and Li2S [36], whereas the cathode peak at 0.01 V is attributed to the intercalation of Li + in the carbon cloth.In the first anode scanning, the anode peak at 0.26 V is related to the deintercalation Li + in the carbon cloth, while the anode peak at 2.19 V is related to the oxidation process of LiS2 to S and Mo to MoS2 [37].The CV curves of the second and third cycles almost coincide with each other, indicating that MoS2/CC-HT has good cyclic repeatability.In the second and third scanning, the cathode peak related to the formation of LixMoS2 by the insertion of Li + in MoS2 layers shifts to 1.93 V; the cathode peak related to the decomposition of LixMoS2 to metallic Mo and Li2S shifts to 1.12 V [38]; the anode peaks at 0.01, 0.21, and 0.82 V can be assigned to the Li + insertion of the carbon cloth.8b) can be calculated by removing the capacity of the carbon cloth.The capacity of MoS2 in MoS2/CC-HT continuously increases to 1075 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles, while the capacity of MoS2 in MoS2/CC-No HT continuously decreases to 528 mAh g −1 .The MoS2 in MoS2/CC No HT is 1T phase, while the MoS2 in MoS2/CC HT is the 2H phase; 2H phase MoS2 has a better capacity retention performance than 1T phase MoS2.Therefore, with heat treatment, MoS2 on the surface of the carbon cloth is transformed from the 1T phase to the 2H phase, resulting in the better capacity retention performance of MoS2/CC-HT.The rate performance of MoS 2 /CC-HT, MoS 2 /CC-No HT, and the carbon cloth are demonstrated in Figure 9a.By removing the capacity of the carbon cloth, the rate performance of MoS 2 in MoS 2 /CC-HT and MoS 2 in MoS 2 /CC-No HT is presented in Figure 9b.MoS 2 in MoS 2 /CC-HT delivers the capacities of 775, 802, 826, 790, and 663 mAh g −1 at current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 , demonstrating its good rate performance.When the current density returns to 0.1 A g −1 , it delivers a capacity of 991 mAh g −1 , which is higher than its initial capacity at the current density of 0.1 A g −1 .MoS 2 in MoS 2 /CC-No HT delivers the capacities of 902, 652, 550, 453, and 311 mAh g −1 at the current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 .When the current density returns to 0.1 A g −1 , it delivers the capacity of 654 mAh g −1 , which is lower than its initial capacity at the current density of 0.     powders.From Figure 10a, it can be seen that MoS2-HT powder has a better capacity retention performance than MoS2-No HT powder.MoS2-HT delivers a capacity of 401 mAh g −1 , while MoS2-No HT delivers a capacity of 258 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles.During charge-discharge, 1T phase MoS2-No HT undergoes a larger volume change than 2H phase MoS2-HT, which leads to its worse capacity retention performance.From Figure 10b, at the gradually increasing current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 , MoS2-HT delivers the capacities of 709, 616, 601, 427, and 363 mAh g −1 , while MoS2-No HT delivers the capacities of 746, 578, 128, 57, and 27 mAh g −1 .

15 Figure 1 .
Figure1shows the SEM images of MoS 2 /CC-HT synthesized under different S/Mo molar ratios.The S/Mo molar ratio has a significant effect on the morphology of MoS 2 /CC-HT.As exhibited in Figure1a,b, it can be seen that the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 5 is ultra-thin nanosheets.In Figure1c, the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 6 shows a similar morphology to the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 5. From Figure 1d, it can be observed that the MoS 2 in MoS 2 /CC-HT synthesized under the S/Mo molar ratio of 4 is composed of particles with the size of 50-200 nm and flower-like nanostructure assembled by nanosheets.Ultra-thin nanosheets will provide many electrochemical reaction sites, decrease the diffusion distance of lithium ions, and reduce the absolute volume change caused by lithiation-delithiation due to their large specific surface area and ultra-thin thickness.Therefore, in the following results and discussion, MoS 2 /CC-HT and MoS 2 /CC-No HT were synthesized under the S/Mo molar ratio of 5.For comparison, MoS 2 -HT and MoS 2 -No HT were also synthesized under the S/Mo molar ratio of 5. MoS 2 /CC-HT and MoS 2 /CC-No HT have the same morphology.Heat treatment at 500 • C only changes the phase of MoS 2 /CC-No HT (see the XRD analysis in Figure 2), not its morphology.Materials 2023, 16, x FOR PEER REVIEW 4 of 15

Figure 2
Figure 2 illustrates the XRD patterns for MoS2/CC-HT and MoS2/CC-No HT.Two peaks at 25.6 and 43.6° correspond to the (002) and (100) crystal planes of graphitic carbon, respectively.These characteristic XRD peaks are from the carbon cloth [23-25].In the XRD pattern of MoS2/CC-No HT, the peak detected at 9.4° is assigned to the (002) crystal plane of 1T phase MoS2 [26,27].The peak intensity is weak, indicating the low crystallinity of the MoS2.

Figure 2 .
Figure 2. XRD patterns of MoS2/CC-HT and MoS2/CC-No HT.X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical states and surface elemental composition of MoS2/CC-HT.As shown in Figure 3a, the XPS survey spectrum of MoS2/CC-HT, the spectrum exhibits characteristic peaks of Mo 3d, S 2p, C 1s, and O 1s, indicating the existence of MoS2 and carbon in MoS2/CC-HT.The characteristic peaks of O 1s are attributed to the water absorbed on the surface of MoS2/CC-HT.The center, FWHM, and area percentage for the deconvoluted peaks of Mo 3d, S 2p, and C 1s are presented in Tabs.S1 to S3, respectively (Supplementary Materials).Figure3bshows the XPS high-resolution spectrum of Mo 3d.The peaks at 232.76 and 229.61 eV are related to Mo 3d3/2 and Mo 3d5/2 of Mo 4+ in MoS2, respectively[28,29]; the peaks at 234.03 and 230.98 eV assigned to Mo 3d3/2 and Mo 3d5/2 of Mo 6+ are related to the Mo-O-C bond between carbon fibers and MoS2 nanosheets[23,30]; the peak at 226.78 eV corresponds to S 2s in MoS2.Figure3cshows the high-resolution XPS spectrum of S 2p; the separated peaks at 163.59 and 162.40 eV correspond to S 2p1/2 and S 2p3/2 of S 2-, respectively[31].In Figure3d, the XPS high-resolution spectrum of C 1s exhibited two separated peaks at 284.80 and 285.77eV, which are assigned to C-C and C-O bonds, respectively[28,32].

Figure 2
Figure 2 illustrates the XRD patterns for MoS 2 /CC-HT and MoS 2 /CC-No HT.Two peaks at 25.6 and 43.6 • correspond to the (002) and (100) crystal planes of graphitic carbon, respectively.These characteristic XRD peaks are from the carbon cloth[23][24][25].In the XRD pattern of MoS 2 /CC-No HT, the peak detected at 9.4 • is assigned to the (002) crystal plane of 1T phase MoS 2[26,27].The peak intensity is weak, indicating the low crystallinity of the MoS 2 .In the XRD pattern of MoS 2 /CC-HT, the peaks observed at 13.8, 33.6, and 59.5 • are attributed to the (002), (100), and (110) crystal planes, respectively, of 2H phase MoS 2 (JCPDS No. 37-1492).The increase in peak intensity indicates an increase in the crystallinity of the 2H phase MoS 2 .According to the Scherrer equation, the average grain size of the 2H phase MoS 2 is 9.9 nm.The grain boundaries are rich in defects which provide channels for the diffusion of lithium ions.Thus, the ultrafine average grain size is conducive to the diffusion of lithium ions in the electrode.X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical states and surface elemental composition of MoS 2 /CC-HT.As shown in Figure3a, the XPS survey spectrum of MoS 2 /CC-HT, the spectrum exhibits characteristic peaks of Mo 3d, S 2p, C 1s, and O 1s, indicating the existence of MoS 2 and carbon in MoS 2 /CC-HT.The characteristic peaks of O 1s are attributed to the water absorbed on the surface of MoS 2 /CC-HT.The center, FWHM, and area percentage for the deconvoluted peaks of Mo 3d, S 2p, and C 1s are presented in Tabs.S1 to S3, respectively (Supplementary Materials).Figure3bshows the XPS high-resolution spectrum of Mo 3d.The peaks at 232.76 and 229.61 eV are related to Mo 3d 3/2 and Mo 3d 5/2 of Mo 4+ in MoS 2 , respectively[28,29]; the peaks at 234.03 and 230.98 eV assigned to Mo 3d 3/2 and Mo 3d 5/2 of Mo 6+ are related to the Mo-O-C bond between carbon fibers and MoS 2 nanosheets[23,30]; the peak at 226.78 eV corresponds to S 2s in MoS 2 .Figure3cshows the high-resolution XPS spectrum of S 2p; the separated peaks at 163.59 and 162.40 eV correspond to S 2p 1/2 and S 2p 3/2 of S 2− , respectively[31].In Figure3d, the XPS high-resolution spectrum of C 1s exhibited two separated peaks at 284.80 and 285.77eV, which are assigned to C-C and C-O bonds, respectively[28,32].
demonstrates the HRTEM image of MoS2.It can be seen that its lattice stripes are irregular.The crystal plane spacing of ~0.27 nm corresponds to the (100) crystal plane of 2H phase MoS2 (JCPDS No. 37-1492).The SAED image of MoS2 presented in Figure 4d illustrates its polycrystalline characteristics.The diffraction rings can be labeled as the (100) and (110) crystal planes of 2H phase MoS2 (JCPDS No. 37-1492).

FigureFigure 6 .
Figure 7a demonstrates the charge-discharge curves of the carbon cloth.The discharge capacities of the carbon cloth are 141, 122, and 114 mAh g −1 in the first three cycles; Figure 6.CV curves of: (a) carbon cloth; (b) MoS 2 /CC-HT.

Figure
Figure 7a demonstrates the charge-discharge curves of the carbon cloth.The discharge capacities of the carbon cloth are 141, 122, and 114 mAh g −1 in the first three cycles; its charge capacities are 129, 120, and 113 mAh g −1 ; and its Coulombic efficiencies are 91.5%,98.4%, and 99.1%.In Figure 7b, the charge-discharge profiles of MoS 2 /CC-HT and the charge voltage plateau at 2.19 V corresponds to the oxidation of LiS 2 to S and Mo to MoS 2 .The discharge capacities of MoS 2 /CC-HT are 1478, 1296, and 1245 mAh g −1 in the first three cycles; its charge capacities are 1370, 1274, and 1234 mAh g −1 ; and its Coulombic efficiencies are 92.7%,98.3%, and 99.1%.Materials 2023, 16, x FOR PEER REVIEW 9 of 15

Figure 7 .
Figure 7. Charge-discharge profiles of: (a) carbon cloth; (b) MoS2/CC-HT.The cyclic performance of MoS2/CC-HT, MoS2/CC-No HT, and the carbon cloth are shown in Figure 8a.The carbon cloth delivers a stable capacity of 110 mAh g −1 during cycling.The capacities of MoS2/CC-HT and MoS2/CC-No HT include the capacity of the carbon cloth and the capacity of MoS2 loaded on the surface of the carbon cloth.Therefore, the capacity of MoS2 in MoS2/CC-No HT and MoS2 in MoS2/CC-No HT (shown in Figure 8b) can be calculated by removing the capacity of the carbon cloth.The capacity of MoS2 in MoS2/CC-HT continuously increases to 1075 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles, while the capacity of MoS2 in MoS2/CC-No HT continuously decreases to 528 mAh g −1 .The MoS2 in MoS2/CC No HT is 1T phase, while the MoS2 in MoS2/CC HT is the 2H phase; 2H phase MoS2 has a better capacity retention performance than 1T phase MoS2.Therefore, with heat treatment, MoS2 on the surface of the carbon cloth is transformed from the 1T phase to the 2H phase, resulting in the better capacity retention performance of MoS2/CC-HT.

Figure 7 .
Figure 7. Charge-discharge profiles of: (a) carbon cloth; (b) MoS 2 /CC-HT.The cyclic performance of MoS 2 /CC-HT, MoS 2 /CC-No HT, and the carbon cloth are shown in Figure 8a.The carbon cloth delivers a stable capacity of 110 mAh g −1 during cycling.The capacities of MoS 2 /CC-HT and MoS 2 /CC-No HT include the capacity of the carbon cloth and the capacity of MoS 2 loaded on the surface of the carbon cloth.Therefore, the capacity of MoS 2 in MoS 2 /CC-No HT and MoS 2 in MoS 2 /CC-No HT (shown in Figure 8b) can be calculated by removing the capacity of the carbon cloth.The capacity of MoS 2 in MoS 2 /CC-HT continuously increases to 1075 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles, while the capacity of MoS 2 in MoS 2 /CC-No HT continuously decreases to 528 mAh g −1 .The MoS 2 in MoS 2 /CC No HT is 1T phase, while the MoS 2 in MoS 2 /CC HT is the 2H phase; 2H phase MoS 2 has a better capacity retention performance than 1T phase MoS 2 .Therefore, with heat treatment, MoS 2 on the surface of the carbon cloth is transformed from the 1T phase to the 2H phase, resulting in the better capacity retention performance of MoS 2 /CC-HT.

Figure 8 .
Figure 8. Cyclic performance: (a) MoS2/CC-HT, MoS2/CC-No HT, and carbon cloth; (b) MoS2 in MoS2/CC-HT and MoS2 in MoS2/CC-No HT.The rate performance of MoS2/CC-HT, MoS2/CC-No HT, and the carbon cloth are demonstrated in Figure 9a.By removing the capacity of the carbon cloth, the rate
1 A g −1 .The larger volume change of 1T phase MoS 2 during charge-discharge leads to the worse capacity retention performance of MoS 2 /CC-No HT. Materials 2023, 16, x FOR PEER REVIEW 10 of 15 performance of MoS2 in MoS2/CC-HT and MoS2 in MoS2/CC-No HT is presented in Figure 9b.MoS2 in MoS2/CC-HT delivers the capacities of 775, 802, 826, 790, and 663 mAh g −1 at current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 , demonstrating its good rate performance.When the current density returns to 0.1 A g −1 , it delivers a capacity of 991 mAh g −1 , which is higher than its initial capacity at the current density of 0.1 A g −1 .MoS2 in MoS2/CC-No HT delivers the capacities of 902, 652, 550, 453, and 311 mAh g −1 at the current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 .When the current density returns to 0.1 A g −1 , it delivers the capacity of 654 mAh g −1 , which is lower than its initial capacity at the current density of 0.1 A g −1 .The larger volume change of 1T phase MoS2 during charge-discharge leads to the worse capacity retention performance of MoS2/CC-No HT.

Figure 10
Figure 10 demonstrates the cyclic and rate performance of MoS2-HT and MoS2-No HT powders.From Figure 10a, it can be seen that MoS2-HT powder has a better capacity retention performance than MoS2-No HT powder.MoS2-HT delivers a capacity of 401 mAh g −1 , while MoS2-No HT delivers a capacity of 258 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles.During charge-discharge, 1T phase MoS2-No HT undergoes a larger volume change than 2H phase MoS2-HT, which leads to its worse capacity retention performance.From Figure 10b, at the gradually increasing current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 , MoS2-HT delivers the capacities of 709, 616, 601, 427, and 363 mAh g −1 , while MoS2-No HT delivers the capacities of 746, 578, 128, 57, and 27 mAh g −1 .

Figure 10
Figure 10 demonstrates the cyclic and rate performance of MoS 2 -HT and MoS 2 -No HT powders.From Figure 10a, it can be seen that MoS 2 -HT powder has a better capacity retention performance than MoS 2 -No HT powder.MoS 2 -HT delivers a capacity of 401 mAh g −1 , while MoS 2 -No HT delivers a capacity of 258 mAh g −1 at a current density of 0.1 A g −1 after 50 cycles.During charge-discharge, 1T phase MoS 2 -No HT undergoes a larger volume change than 2H phase MoS 2 -HT, which leads to its worse capacity retention performance.From Figure 10b, at the gradually increasing current densities of 0.1, 0.2, 0.5, 1, and 2 A g −1 , MoS 2 -HT delivers the capacities of 709, 616, 601, 427, and 363 mAh g −1 , while MoS 2 -No HT delivers the capacities of 746, 578, 128, 57, and 27 mAh g −1 .

Figure
Figure11ashows the XRD patterns of MoS2-HT and MoS2-No HT powders.It can be seen that MoS2-HT is 2H phase, while MoS2-No HT is 1T phase, indicating that the carbon cloth substrate will not change the phase of the loaded MoS2.Figure11b,c demonstrate

Figure
Figure 11a shows the XRD patterns of MoS 2 -HT and MoS 2 -No HT powders.It can be seen that MoS 2 -HT is 2H phase, while MoS 2 -No HT is 1T phase, indicating that the carbon cloth substrate will not change the phase of the loaded MoS 2 .Figure 11b,c demonstrate the SEM image of MoS 2 -No HT and MoS 2 -HT powder, respectively.Because heat treatment only changes the phase of MoS 2 , MoS 2 -No HT powder exhibits the same morphology as MoS 2 -HT powder.From Figure 11b,c, it can be seen that both MoS 2 -No HT powder and MoS 2 -HT powder consist of MoS 2 flowers (area outside the red ring in Figure 11b,c) and MoS 2 aggregates (area inside the red ring in Figure 11b,c).Because MoS 2 in this paper is synthesized by a hydrothermal reaction in an aqueous solution of ammonium molybdate and thiourea, the strong polarity of water inevitably leads to the aggregation of the synthesized MoS 2 .Large-sized MoS 2 aggregates will undergo fragmentation, pulverization, and detachment during the charge-discharge, resulting in their capacity fading.The carbon cloth substrate causes the loaded MoS 2 to form a mutually separated nanosheet array.The non-agglomerated MoS 2 nanosheet array will increase electrochemical reaction sites and promote the diffusion of lithium ions in the electrode.Thus, MoS 2 /CC-HT delivers higher capacity than MoS 2 -HT, while MoS 2 /CC-No HT delivers higher capacity than MoS 2 -No HT. Materials 2023, 16, x FOR PEER REVIEW 11 of 15

Figure 12 Figure 11 .
Figure 12 presents the EIS patterns of MoS2/CC-HT, MoS2/CC-No HT, MoS2-HT, and MoS2-No HT.In the mid-frequency region, the diameter of the semicircle in the Nyquist plot is related to the charge transfer resistance of the electrode [39,40].The larger semicircle diameter means the larger charge transfer resistance of the electrode.The charge transfer resistances of MoS2/CC-HT, MoS2/CC-No HT, MoS2-HT, and MoS2-No HT are 60.2, 245.6, Figure 11.(a) XRD patterns of MoS 2 -HT and MoS 2 -No HT; SEM images of (b) MoS 2 -No HT and (c) MoS 2 -HT.

Figure 12
Figure 12 presents the EIS patterns of MoS 2 /CC-HT, MoS 2 /CC-No HT, MoS 2 -HT, and MoS 2 -No HT.In the mid-frequency region, the diameter of the semicircle in the Nyquist plot is related to the charge transfer resistance of the electrode [39,40].The larger semicircle diameter means the larger charge transfer resistance of the electrode.The charge transfer resistances of MoS 2 /CC-HT, MoS 2 /CC-No HT, MoS 2 -HT, and MoS 2 -No HT are 60.2, 245.6, 466.8, and 575.5 Ω, respectively.The carbon cloth has good conductivity.The conductivity of the loaded MoS 2 nanosheets can be enhanced by the carbon cloth substrate, thereby reducing their charge transfer resistance.In the low-frequency region, the slope of the oblique line in the Nyquist plot is related to the diffusion rate of lithium ions in the electrode.The steeper the slope of the oblique line, the faster the rate of diffusion of lithium ions into the electrode [41].Apparently, the diffusion rate of lithium ions in MoS 2 /CC-HT is faster than that in MoS 2 -HT, and the diffusion rate of lithium ions in MoS 2 /CC-No HT is faster than that in MoS 2 -No HT.The mutually separated MoS 2 nanosheets grown on the surface of the carbon cloth will promote the diffusion of lithium ions in the electrode.Materials 2023, 16, x FOR PEER REVIEW 12 of 15

Figure
Figure 13a,b provide the SEM images of MoS2/CC-HT after 50 cycles at a current density of 0.1 A g −1 .The MoS2 layer still firmly adhered to the surface of the carbon cloth without shedding.In contrast, for MoS2/CC-No HT after cycling, a large amount of detachment occurred in the MoS2 layer (Figure 13c,d).This is the reason why MoS2/CC-HT has a better capacity retention performance than MoS2/CC-No HT.

Figure
Figure 13a,b provide the SEM images of MoS 2 /CC-HT after 50 cycles at a current density of 0.1 A g −1 .The MoS 2 layer still firmly adhered to the surface of the carbon cloth without shedding.In contrast, for MoS 2 /CC-No HT after cycling, a large amount of detachment occurred in the MoS 2 layer (Figure 13c,d).This is the reason why MoS 2 /CC-HT has a better capacity retention performance than MoS 2 /CC-No HT.